Silk is a natural proteinfiber, some forms of which can be woven into textiles. The protein fiber of silk is composed mainly of fibroin and is produced by certain insect larvae to form cocoons.[1] The best-known silk is obtained from the cocoons of the larvae of the mulberrysilkwormBombyx mori reared in captivity (sericulture). The shimmering appearance of silk is due to the triangular prism-like structure of the silk fibre, which allows silk cloth to refract incoming light at different angles, thus producing different colors.

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✪ The Silk Road and Ancient Trade: Crash Course World History #9

Transcription

Hi there, I’m John Green, you’re watching
Crash Course: World History, and today we’re
gonna talk about the Silk Road, so called
because it was not a road and not made of
silk.
So this is a t-shirt. It was designed in Belgium
and contains cotton from both Brazil and the
Texas, which was turned into cloth in China,
stitched in Haiti, screen-printed in the Washington,
sold to me in Indiana, and now that I am too
fat to wear it, it will soon make its way
to Cameroon or Honduras or possibly even back
to Haiti.
Can we just pause for a moment to consider
the astonishing fact that most t-shirts see
more of the World than most of us do—
Mr. Green Mr. Green the t-shirt can’t see
the world because they don’t have eyes—
Look, me from the past, it’s difficult for
me to isolate what I hate most about you because
there is so much to hate.
But very near the top is your relentless talent
for ignoring everything that is interesting
and beautiful about our species in favor of
pedantic sniveling in which no one loses or
gains anything of value.
I’m gonna go put on a collared shirt because
we’re here to tackle the big picture.
[music intro]
[music intro]
[music intro]
[music intro]
[music intro]
[music intro]
So the silk road didn’t begin trade, but
it did radically expand its scope, and the
connections that were formed by mostly unknown
merchants arguably changed the world more
than any political or religious leaders.
It was especially cool If you were rich, because
you finally had something to spend your money
on other than temples. But even if you weren’t
rich, the Silk Road reshaped the lives of
everyone living in Africa and Eurasia, as
we will see today. Let’s go straight to
the Thought Bubble.
As previously mentioned, the silk road was
not a road. It’s not like archaeologists
working in Uzbekistan have uncovered a bunch
of yield signs and baby on board stickers.
It was an overland route where merchants carried
goods for trade.
But it was really two routes: One that connected
the Eastern Mediterranean to Central Asia
and one that went from Central Asia to China.
Further complicating things, the Silk Road
involved sea routes: Many goods reached Rome
via the Mediterranean,
and goods from Central Asia found their way
across the Pacific to Japan and even Java.
So we shouldn’t think of the Silk Road as
a road but rather as a network of trade routes.
But just as now, the goods traveled more than
the people who traded them: Very few traders
traversed the entire silk road: Instead, they’d
move back and forth between towns, selling
to traders who’d take the goods further
toward their destination, with everybody marking
up prices along the way.
So what’d they trade? Well silk, for starters.
For millennia, silk was only produced in China.
It is spun from the cocoons of mulberry tree-eating
worms and the process of silk making as well
as the techniques for raising the worms were
closely guarded secrets, since the lion’s
share of China’s wealth came from silk production.
The Chinese used silk as fishing line, to
buy off nomadic raiders to keep things peaceful,
and to write before they invented paper.
But as an export, silk was mostly used for
clothes: Silk clothing feels light in the
summer and warm in the winter, and until we
invented $700 pre-distressed designer jeans,
decking yourself out in silk was the #1 way
to show people that you were wealthy.
Thanks, Thought Bubble. But the silk road
wasn’t all about silk.
The Mediterranean exported such cliched goods
as olives, olive oil, wine, and mustachioed
plumbers.
China exported raw materials like jade, silver,
and iron.
India exported fine cotton textiles; the ivory
that originated in East Africa made its way
across the Silk Road;
And Arabia exported incense and spices and
tortoise shells. Oh, god, it’s a red one,
isn’t it? It’s just gonna chase me, I
just--- Ow.
Up until now on Crash Course we’ve been
focused on city-dwelling civilizational types,
but with the growth of the silk road, the
nomadic people of Central Asia suddenly become
much more important to world history.
Much of Central Asia isn’t great for agriculture,
but it’s difficult to conquer, unless you
are, wait for it- The Mongols.
It also lends itself fairly well to herding,
and since nomads are definitionally good at
moving around, they’re also good at moving
stuff from Point A to Point B, which makes
them good traders.
Plus all their travel made them more resistant
to diseases.
One group of such nomads, the Yuezhi, were
humiliated in battle in the 2nd century BCE
by their bitter rivals the Xiongnu, who turned
the Yuezhi king’s skull into a drinking
cup, in fact.
And in the wake of that the Yuezhi migrated
to Bactria and started the Kushan Empire in
what is now Afghanistan and Pakistan.
Although silk road trading began more than
a century before the birth of Jesus, it really
took off in the second and third centuries
CE, and the Kushan Empire became a huge hub
for that silk road trade.
By then, nomads were being eclipsed by professional
merchants who travelled the silk roads, often
making huge profits, but those cities that
had been founded by nomadic peoples became
hugely important.
They continued to grow, because most of the
trade on the Silk Road was by caravan, and
those caravans had to stop frequently, you
know, for like food and water and prostitutes.
These towns became fantastically wealthy:
One, Palmyra, was particularly important because
all of the incense and silk that travelled
to Rome had to go through Palmyra.
Silk was so popular among the Roman elite
that the Roman senate repeatedly tried to
ban it, complaining about trade imbalances
caused by the silk trade and also that silk
was inadequately modest.
To quote Seneca the Younger,
“I see clothes of silk, if materials that
do not hide the body, nor even one's decency,
can be called clothes,”
he also said of the woman who wears silk,
“her husband has no more acquaintance than
any outsider or foreigner with his wife's
body."
And yet all attempts to ban silk failed, which
speaks to how much, even in the ancient world,
wealth shaped governance.
And with trade, there was a way to become
wealthy without being a king or lord who takes
part of what your citizens produce.
The merchant class that grew along with the
Silk Road came to have a lot of political
clout, and in some ways that began the tension
that we still see today between wealth and
politics. Whether it’s, you know, corporations
making large donations or Vladimir Putin periodically
jailing billionaires.
Mr. Putin, I just want to state for the record
that I did not mean that in any way, I was---
Stan wrote that joke.
Oh, it’s time for the Open Letter.
An Open Letter to Billionaires:
But first, let’s see what’s in the Secret
Compartment today. Oh, it’s some fake silk;
the stuff that put real silk out of business.
Dear Billionaires,
I’ve wrapped myself in the finest of polyester
so that you will take my message seriously.
Here at Crash Course we’ve done a lot of
research into our demographics and our show
is watched primarily by Grammar Nazis, Muggle
Quidditch Players, People Who Have a Test
Tomorrow, and Billionaires.
I have a message for you Billionaires: It
will never be enough. You’re relentless
yearning is going to kill us all.
Best wishes,
John Green
Speaking of billionaires, the goods that travelled
on the Silk Road really only changed the lives
of rich people. Did the Silk Road affect the
rest of us? Yes, for three reasons.Second,
the Silk Road didn’t just trade luxury goods.
In fact, arguably the most important thing
traded along the Silk Road: ideas.
First, wider economic impact. Relatively few
people could afford silk, but a lot of people
devoted their lives to making that silk.
And as the market for silk grew, more and
more people chose to go into silk production
rather than doing something else with their
lives.
Second, the Silk Road didn’t just trade
luxury goods. In fact, arguably the most important
thing traded along the Silk Road: ideas.
For example, the Silk Road was the primary
route for the spread of Buddhism.When we last
saw the Buddha’s Eight-Fold Path to escaping
the cycle of suffering and desire that's inherent
to humans, it was beginning to dwindle in
India.
But through contacts with other cultures and
traditions, Buddhism grew and flourished and
became one of the great religious traditions
of the world.
The variation of Buddhism that took root in
China, Korea, Japan, and Central Asia is known
as Mahayana Buddhism, and it differed from
the original teachings of the Buddha in many
ways, but one that was fundamental. For Mahayana
Buddhists, the Buddha was divine. (I mean,
we can—and religious historians do—fight
over the exact definition of divine, but in
Mahayanna Buddhism, there’s no question
that the Buddha is venerated to a greater
degree.
The idea of Nirvana also transformed from
a release from that cycle of suffering and
desire to something much more heavenly and
frankly more fun, and in some versions of
Mahayana Buddhism, there are lots of different
heavens, each more awesome than the last.
Rather than focusing on the fundamental fact
of suffering, Mahayana Buddhism offered the
hope that through worship of the Buddha, or
one of the many bodhisattvas – holy people
who could have achieved nirvana but chose
to hang out on Earth with us because they’re
super nice– one could attain a good afterlife.
Many merchants on the silk road became strong
supporters of monasteries which in turn became
convenient weigh stations for caravans.
And by endowing the monasteries, rich merchants
were buying a form of supernatural insurance;
Monks who lived in the monasteries would pray
for the success of trade missions and the
health of their patrons. It was win-win, especially
when you consider that one of the central
materials used in Mahayana Buddhist rituals
is … silk.
And a third reason the silk road changed all
our lives, worldwide interconnectedness of
populations led to the spread of disease.
Measles and Smallpox traveled along it, as
did bubonic plague, which came from the East
to the West in 534, 750, and—most devastatingly—in
1346.
This last plague—known as the Black Death—resulted
in the largest population decimation in human
history, with nearly half of Europeans dying
in a four-year period.
A sizable majority of people living in Italy
died as did two-thirds of Londoners.
And it quite possibly wouldn’t have happened
without the Silk Road. If you were living
in London during the fourteenth century, you
probably didn’t blame the Silk Road for
your community’s devastation, but it played
a role.
If you look at it that way, the interconnectedness
fostered by Silk Road affected way, way more
people than just those rich enough to buy
silk, just as today’s globalization offers
both promise and threat to each of us.
Next week we’ll talk about Julius Caesar
and in what situation, if any, it’s okay
to stab your friend in the gut. Until then,
thanks for watching.
Crash Course is produced and directed by Stan
Muller, our script supervisor is Danica Johnson.
Our graphics team is Thought Bubble and the
show is written by my high school history
teacher Raoul Meyer and myself.
Last week's Phrase of the Week was "Kim Kardashian".
If you didn't like it, SUGGEST BETTER PHRASES
OF THE WEEK IN COMMENTS. Every week I take
one of your suggestions and find a way to
squeeze it into the new episode.
If you liked today's episode of Crash Course,
please click the "like" button and consider
sharing the show with your friends.
You can also follow us on Twitter @THECRASHCOURSE
or on Facebook, links below.
Raoul also has a Twitter where he tweets Crash
Course pop quizzes. As do I. All of those
links can be found below.
Also, the beloved and not fictitious, Stan,
has agreed to start tweeting. So that's exciting!
Thanks for watching, and as we say in my hometown,
don’t forget to be awesome.
[scoots out of frame]
[scoots out of frame]
Oh, hey. Remember that Mongols shirt from
the beginning of the episode? In addition
to being a joke, it's a shirt!
So many of you requested Mongols shirts that
WE ARE GIVING THEM TO YOU! [ available for
purchase, rather] They are now available for
pre-order at DFTBA.com, link in the video
info below, so you can show your love for
Crash Course or Mongols or exceptions.

History

Wild silk

Several kinds of wild silk, which are produced by caterpillars other than the mulberry silkworm, have been known and used in China, South Asia, and Europe since ancient times. However, the scale of production was always far smaller than for cultivated silks. There are several reasons for this: first, they differ from the domesticated varieties in colour and texture and are therefore less uniform; second, cocoons gathered in the wild have usually had the pupa emerge from them before being discovered so the silk thread that makes up the cocoon has been torn into shorter lengths; and third, many wild cocoons are covered in a mineral layer that prevents attempts to reel from them long strands of silk.[5] Thus, the only way to obtain silk suitable for spinning into textiles in areas where commercial silks are not cultivated was by tedious and labor-intensive carding.

Commercial silks originate from reared silkworm pupae, which are bred to produce a white-colored silk thread with no mineral on the surface. The pupae are killed by either dipping them in boiling water before the adult moths emerge or by piercing them with a needle. These factors all contribute to the ability of the whole cocoon to be unravelled as one continuous thread, permitting a much stronger cloth to be woven from the silk.
Wild silks also tend to be more difficult to dye than silk from the cultivated silkworm.[6][7] A technique known as demineralizing allows the mineral layer around the cocoon of wild silk moths to be removed,[8] leaving only variability in color as a barrier to creating a commercial silk industry based on wild silks in the parts of the world where wild silk moths thrive, such as in Africa and South America.

Legend gives credit for developing silk to a Chinese empress, Leizu (Hsi-Ling-Shih, Lei-Tzu). Silks were originally reserved for the Emperors of China for their own use and gifts to others, but spread gradually through Chinese culture and trade both geographically and socially, and then to many regions of Asia. Because of its texture and lustre, silk rapidly became a popular luxury fabric in the many areas accessible to Chinese merchants. Silk was in great demand, and became a staple of pre-industrial international trade. In July 2007, archaeologists discovered intricately woven and dyed silk textiles in a tomb in Jiangxi province, dated to the Eastern Zhou Dynasty roughly 2,500 years ago.[14] Although historians have suspected a long history of a formative textile industry in ancient China, this find of silk textiles employing "complicated techniques" of weaving and dyeing provides direct evidence for silks dating before the Mawangdui-discovery and other silks dating to the Han Dynasty (202 BC-220 AD).[14]

Silk is described in a chapter on mulberry planting by Si Shengzhi of the Western Han (206 BC – 9 AD). There is a surviving calendar for silk production in an Eastern Han (25–220 AD) document. The two other known works on silk from the Han period are lost.[9] The first evidence of the long distance silk trade is the finding of silk in the hair of an Egyptianmummy of the 21st dynasty, c.1070 BC.[15] The silk trade reached as far as the Indian subcontinent, the Middle East, Europe, and North Africa. This trade was so extensive that the major set of trade routes between Europe and Asia came to be known as the Silk Road.

In the ancient era, silk from China was the most lucrative and sought-after luxury item traded across the Eurasian continent,[19] and many civilizations, such as the ancient Persians, benefited economically from trade.[19]

Chinese silk making process

The silkworms and mulberry leaves are placed on trays.

Twig frames for the silkworms are prepared.

The cocoons are weighed.

The cocoons are soaked and the silk is wound on spools.

The silk is woven using a loom.

India

Silk has a long history in India. It is known as Resham in eastern and north India, and Pattu in southern parts of India. Recent archaeological discoveries in Harappa and Chanhu-daro suggest that sericulture, employing wild silk threads from native silkworm species, existed in South Asia during the time of the Indus Valley Civilization (now in Pakistan) dating between 2450 BC and 2000 BC, while "hard and fast evidence" for silk production in China dates back to around 2570 BC.[20][21] Shelagh Vainker, a silk expert at the Ashmolean Museum in Oxford, who sees evidence for silk production in China "significantly earlier" than 2500–2000 BC, suggests, "people of the Indus civilization either harvested silkworm cocoons or traded with people who did, and that they knew a considerable amount about silk."[20]

India is also the largest consumer of silk in the world. The tradition of wearing silk sarees for marriages and other auspicious ceremonies is a custom in Assam and southern parts of India. Silk is considered to be a symbol of royalty, and, historically, silk was used primarily by the upper classes. Silk garments and sarees produced in Kanchipuram, Pochampally, Dharmavaram, Mysore, Arani in the south, Banaras in the north, Bhagalpur and Murshidabad in the east are well recognized. In the northeastern state of Assam, three different types of silk are produced, collectively called Assam silk: Muga, Eri and Pat silk. Muga, the golden silk, and Eri are produced by silkworms that are native only to Assam.

Thailand

Silk is produced year-round in Thailand by two types of silkworms, the cultured Bombycidae and wild Saturniidae. Most production is after the rice harvest in the southern and northeastern parts of the country. Women traditionally weave silk on hand looms and pass the skill on to their daughters, as weaving is considered to be a sign of maturity and eligibility for marriage. Thai silk textiles often use complicated patterns in various colours and styles. Most regions of Thailand have their own typical silks. A single thread filament is too thin to use on its own so women combine many threads to produce a thicker, usable fiber. They do this by hand-reeling the threads onto a wooden spindle to produce a uniform strand of raw silk. The process takes around 40 hours to produce a half kilogram of silk. Many local operations use a reeling machine for this task, but some silk threads are still hand-reeled. The difference is that hand-reeled threads produce three grades of silk: two fine grades that are ideal for lightweight fabrics, and a thick grade for heavier material.

The silk fabric is soaked in extremely cold water and bleached before dyeing to remove the natural yellow coloring of Thai silk yarn. To do this, skeins of silk thread are immersed in large tubs of hydrogen peroxide. Once washed and dried, the silk is woven on a traditional hand-operated loom.[25]

Bangladesh

The Rajshahi Division of northern Bangladesh is the hub of the country's silk industry. There are three types of silk produced in the region: mulberry, endi and tassar. Bengali silk was a major item of international trade for centuries. It was known as Ganges silk in medieval Europe. Bengal was the leading exporter of silk between the 16th and 19th centuries.[26]

Ancient Mediterranean

In the Odyssey, 19.233, when Odysseus, while pretending to be someone else, is questioned by Penelope about her husband's clothing, he says that he wore a shirt "gleaming like the skin of a dried onion" (varies with translations, literal translation here)[27] which could refer to the lustrous quality of silk fabric.
Aristotle wrote of Coa vestis, a wild silk textile from Kos.
Sea silk from certain large sea shells was also valued.
The Roman Empire knew of and traded in silk, and Chinese silk was the most highly priced luxury good imported by them.[19] During the reign of emperor Tiberius, sumptuary laws were passed that forbade men from wearing silk garments, but these proved ineffectual.[28] The Historia Augusta mentions that the 3rd Century AD emperor Elagabalus was the first Roman to wear garments of pure silk, whereas it had been customary to wear fabrics of silk/cotton or silk/linen blends.[29] Despite the popularity of silk, the secret of silk-making only reached Europe around AD 550, via the Byzantine Empire. Legend has it that monks working for the emperor Justinian Ismuggled silkworm eggs to Constantinople in hollow canes from China. All top-quality looms and weavers were located inside the Great Palace complex in Constantinople, and the cloth produced was used in imperial robes or in diplomacy, as gifts to foreign dignitaries. The remainder was sold at very high prices.

Middle East

In the Torah, a scarlet cloth item called in Hebrew "sheni tola'at" שני תולעת – literally "crimson of the worm" – is described as being used in purification ceremonies, such as those following a leprosy outbreak (Leviticus 14), alongside cedar wood and hyssop (za'atar). Eminent scholar and leading medieval translator of Jewish sources and books of the Bible into Arabic, Rabbi Saadia Gaon, translates this phrase explicitly as "crimson silk" – חריר קרמז حرير قرمز.

In Islamic teachings, Muslim men are forbidden to wear silk. Many religious jurists believe the reasoning behind the prohibition lies in avoiding clothing for men that can be considered feminine or extravagant.[30] There are disputes regarding the amount of silk a fabric can consist of (e.g., whether a small decorative silk piece on a cotton caftan is permissible or not) for it to be lawful for men to wear, but the dominant opinion of most Muslim scholars is that the wearing of silk by men is forbidden. Modern attire has raised a number of issues, including, for instance, the permissibility of wearing silk neckties, which are masculine articles of clothing.

Despite injunctions against silk for men, silk has retained its popularity in the Islamic world because of its permissibility for women, and due to the presence of non-Muslim communities. The Muslim Moors brought silk with them to Spain during their conquest of the Iberian Peninsula.

Medieval and modern Europe

Silk satin leaf, wood sticks and guards, c. 1890

Italy was the most important producer of silk during the Medieval age. The first center to introduce silk production to Italy was the city of Catanzaro during the 11th century in the region of Calabria. The silk of Catanzaro supplied almost all of Europe and was sold in a large market fair in the port of Reggio Calabria, to Spanish, Venetian, Genovese and Dutch merchants. Catanzaro became the lace capital of the world with a large silkworm breeding facility that produced all the laces and linens used in the Vatican. The city was world-famous for its fine fabrication of silks, velvets, damasks and brocades.[31]

Another notable center was the Italian city-state of Lucca which largely financed itself through silk-production and silk-trading, beginning in the 12th century. Other Italian cities involved in silk production were Genoa, Venice and Florence.

The Silk Exchange in Valencia from the 15th century—where previously in 1348 also perxal (percale) was traded as some kind of silk—illustrates the power and wealth of one of the great Mediterranean mercantile cities.[32][33]

Silk was produced in and exported from the province of Granada, Spain, especially the Alpujarras region, until the Moriscos, whose industry it was, were expelled from Granada in 1571.[34][35]

Since the 15th century, silk production in France has been centered around the city of Lyon where many mechanic tools for mass production were first introduced in the 17th century.

James I attempted to establish silk production in England, purchasing and planting 100,000 mulberry trees, some on land adjacent to Hampton Court Palace, but they were of a species unsuited to the silk worms, and the attempt failed. In 1732 John Guardivaglio set up a silk throwing enterprise at Logwood mill in Stockport; in 1744, Burton Mill was erected in Macclesfield; and in 1753 Old Mill was built in Congleton.[36] These three towns remained the centre of the English silk throwing industry until silk throwing was replaced by silk waste spinning. British enterprise also established silk filature in Cyprus in 1928. In England in the mid-20th century, raw silk was produced at Lullingstone Castle in Kent. Silkworms were raised and reeled under the direction of Zoe Lady Hart Dyke, later moving to Ayot St Lawrence in Hertfordshire in 1956.[37]

North America

Wild silk taken from the nests of native caterpillars was used by the Aztecs to make containers and as paper.[41][42] Silkworms were introduced to Oaxaca from Spain in the 1530s and the region profited from silk production until the early 17th century, when the king of Spain banned export to protect Spain's silk industry. Silk production for local consumption has continued until the present day, sometimes spinning wild silk.[43]

King James I introduced silk-growing to the British colonies in America around 1619, ostensibly to discourage tobacco planting. The Shakers in Kentucky adopted the practice.

The history of industrial silk in the United States is largely tied to several smaller urban centers in the Northeast region. Beginning in the 1830s, Manchester, Connecticut emerged as the early center of the silk industry in America, when the Cheney Brothers became the first in the United States to properly raise silkworms on an industrial scale; today the Cheney Brothers Historic District showcases their former mills.[45] With the mulberry tree craze of that decade, other smaller producers began raising silkworms. This economy particularly gained traction in the vicinity of Northampton, Massachusetts and its neighboring Williamsburg, where a number of small firms and cooperatives emerged. Among the most prominent of these was the cooperative utopian Northampton Association for Education and Industry, of which Sojourner Truth was a member.[46] Following the destructive Mill River Flood of 1874, one manufacturer, William Skinner, relocated his mill from Williamsburg to the then-new city of Holyoke. Over the next 50 years he and his sons would maintain relations between the American silk industry and its counterparts in Japan,[47] and expanded their business to the point that by 1911, the Skinner Mill complex contained the largest silk mill under one roof in the world, and the brand Skinner Fabrics had become the largest manufacturer of silk satins internationally.[44][48] Other efforts later in the 19th century would also bring the new silk industry to Paterson, New Jersey, with several firms hiring European-born textile workers and granting it the nickname "Silk City" as another major center of production in the United States.

World War II interrupted the silk trade from Asia, and silk prices increased dramatically.[49] U.S. industry began to look for substitutes, which led to the use of synthetics such as nylon. Synthetic silks have also been made from lyocell, a type of cellulose fiber, and are often difficult to distinguish from real silk (see spider silk for more on synthetic silks).

Malaysia

In Terengganu, which is now part of Malaysia, a second generation of silkworm was being imported as early as 1764 for the country's silk textile industry, especially songket.[50] However, since the 1980s, Malaysia is no longer engaged in sericulture but does plant mulberry trees.

Vietnam

In Vietnamese legend, silk appeared in the sixth dynasty of Hùng Vương.

Production process

The process of silk production is known as sericulture.[51] The entire production process of silk can be divided into several steps which are typically handled by different entities[clarification needed]. Extracting raw silk starts by cultivating the silkworms on mulberry leaves. Once the worms start pupating in their cocoons, these are dissolved in boiling water in order for individual long fibres to be extracted and fed into the spinning reel.[52]

To produce 1 kg of silk, 104 kg of mulberry leaves must be eaten by 3000 silkworms. It takes about 5000 silkworms to make a pure silk kimono.[53]:104 The major silk producers are China (54%) and India (14%).[54] Other statistics:[55]

The environmental impact of silk production is potentially large when compared with other natural fibers. A life cycle assessment of Indian silk production shows that the production process has a large carbon and water footprint, mainly due to the fact that it is an animal-derived fiber and more inputs such as fertilizer and water are needed per unit of fiber produced.[56]

Properties

Models in silk dresses at the MoMo Falana fashion show

Physical properties

Silk fibers from the Bombyx mori silkworm have a triangularcross section with rounded corners, 5–10 μm wide. The fibroin-heavy chain is composed mostly of beta-sheets, due to a 59-mer amino acid repeat sequence with some variations.[57] The flat surfaces of the fibrils reflect light at many angles, giving silk a natural sheen. The cross-section from other silkworms can vary in shape and diameter: crescent-like for Anaphe and elongated wedge for tussah. Silkworm fibers are naturally extruded from two silkworm glands as a pair of primary filaments (brin), which are stuck together, with sericin proteins that act like glue, to form a bave. Bave diameters for tussah silk can reach 65 μm. See cited reference for cross-sectional SEM photographs.[58]

Raw silk of domesticated silk worms, showing its natural shine.

Silk has a smooth, soft texture that is not slippery, unlike many synthetic fibers.

Silk is one of the strongest natural fibers, but it loses up to 20% of its strength when wet. It has a good moisture regain of 11%. Its elasticity is moderate to poor: if elongated even a small amount, it remains stretched. It can be weakened if exposed to too much sunlight. It may also be attacked by insects, especially if left dirty.

One example of the durable nature of silk over other fabrics is demonstrated by the recovery in 1840 of silk garments from a wreck of 1782: 'The most durable article found has been silk; for besides pieces of cloaks and lace, a pair of black satin breeches, and a large satin waistcoat with flaps, were got up, of which the silk was perfect, but the lining entirely gone ... from the thread giving way ... No articles of dress of woollen cloth have yet been found.'[59]

Silk is a poor conductor of electricity and thus susceptible to static cling. Silk has a high emissivity for infrared light, making it feel cool to the touch.[60]

Unwashed silk chiffon may shrink up to 8% due to a relaxation of the fiber macrostructure, so silk should either be washed prior to garment construction, or dry cleaned. Dry cleaning may still shrink the chiffon up to 4%. Occasionally, this shrinkage can be reversed by a gentle steaming with a press cloth. There is almost no gradual shrinkage nor shrinkage due to molecular-level deformation.

Natural and synthetic silk is known to manifest piezoelectric properties in proteins, probably due to its molecular structure.[61]

Silkworm silk was used as the standard for the denier, a measurement of linear density in fibers. Silkworm silk therefore has a linear density of approximately 1 den, or 1.1 dtex.

Chemical properties

Silk emitted by the silkworm consists of two main proteins, sericin and fibroin, fibroin being the structural center of the silk, and serecin being the sticky material surrounding it. Fibroin is made up of the amino acidsGly-Ser-Gly-Ala-Gly-Ala and forms beta pleated sheets. Hydrogen bonds form between chains, and side chains form above and below the plane of the hydrogen bond network.

The high proportion (50%) of glycine allows tight packing. This is because glycine's R group is only a hydrogen and so is not as sterically constrained. The addition of alanine and serine makes the fibres strong and resistant to breaking. This tensile strength is due to the many interceded hydrogen bonds, and when stretched the force is applied to these numerous bonds and they do not break.

Silk is resistant to most mineral acids, except for sulfuric acid, which dissolves it. It is yellowed by perspiration. Chlorine bleach will also destroy silk fabrics.

Variants

Regenerated silk fiber

RSF is produced by chemically dissolving silkworm cocoons, leaving their molecular structure intact. The silk fibers dissolve into tiny thread-like structures known as microfibrils. The resulting solution is extruded through a small opening, causing the microfibrils to reassemble into a single fiber. The resulting material is reportedly twice as stiff as silk.[63]

Furniture

Silk's attractive lustre and drape makes it suitable for many furnishing applications. It is used for upholstery, wall coverings, window treatments (if blended with another fiber), rugs, bedding and wall hangings.[citation needed]

Industry

Medicine

A special manufacturing process removes the outer sericin coating of the silk, which makes it suitable as non-absorbable surgical sutures. This process has also recently led to the introduction of specialist silk underclothing, which has been used for skin conditions including eczema.[65][66] New uses and manufacturing techniques have been found for silk for making everything from disposable cups to drug delivery systems and holograms.[67]

Biomaterial

Silk has been considered as a luxurious textile since 3630 BC. However, it started to serve also as a biomedical material for suture in surgeries decades ago. In the past 30 years, it has been widely studied and used as a biomaterial, which refers to materials used for medical applications in organisms, due to its excellent properties, including remarkable mechanical properties, comparative biocompatibility, tunable degradation rates in vitro and in vivo, the ease to load cellular growth factors (for example, BMP-2), and the ability to be processed into several other formats such as films, gels, particles, and scaffolds.[68] Silks from Bombyx mori, a kind of cultivated silkworm, are the most widely investigated silks.[69]

Silks derived from Bombyx mori are generally made of two parts: the silk fibroin fiber which contains a light chain of 25kDa and a heavy chain of 350kDa (or 390kDa[70]) linked by a single disulfide bond[71] and a glue-like protein, sericin, comprising 25 to 30 percentage by weight. Silk fibroin contains hydrophobic beta sheet blocks, interrupted by small hydrophilic groups. And the beta-sheets contribute much to the high mechanical strength of silk fibers, which achieves 740 MPa, tens of times that of poly(lactic acid) and hundreds of times that of collagen. This impressive mechanical strength has made silk fibroin very competitive for applications in biomaterials. Indeed, silk fibers have found their way into tendon tissue engineering,[72] where mechanical properties matter greatly. In addition, mechanical properties of silks from various kinds of silkworms vary widely, which provides more choices for their use in tissue engineering.

Most products fabricated from regenerated silk are weak and brittle, with only ≈1–2% of the mechanical strength of native silk fibers due to the absence of appropriate secondary and hierarchical structure,

Biocompatibility

Biocompatibility, i.e., to what level the silk will cause an immune response, is a critical issue for biomaterials. The issue arose during its increasing clinical use. Wax or silicone is usually used as a coating to avoid fraying and potential immune responses[68] when silk fibers serve as suture materials. Although the lack of detailed characterization of silk fibers, such as the extent of the removal of sericin, the surface chemical properties of coating material, and the process used, make it difficult to determine the real immune response of silk fibers in literature, it is generally believed that sericin is the major cause of immune response. Thus, the removal of sericin is an essential step to assure biocompatibility in biomaterial applications of silk. However, further research fails to prove clearly the contribution of sericin to inflammatory responses based on isolated sericin and sericin based biomaterials.[74] In addition, silk fibroin exhibits an inflammatory response similar to that of tissue culture plastic in vitro[75][76] when assessed with human mesenchymal stem cells (hMSCs) or lower than collagen and PLA when implant rat MSCs with silk fibroin films in vivo.[76] Thus, appropriate degumming and sterilization will assure the biocompatibility of silk fibroin, which is further validated by in vivo experiments on rats and pigs.[77] There are still concerns about the long-term safety of silk-based biomaterials in the human body in contrast to these promising results. Even though silk sutures serve well, they exist and interact within a limited period depending on the recovery of wounds (several weeks), much shorter than that in tissue engineering. Another concern arises from biodegradation because the biocompatibility of silk fibroin does not necessarily assure the biocompatibility of the decomposed products. In fact, different levels of immune responses[78][79] and diseases[80] have been triggered by the degraded products of silk fibroin.

Biodegradability

Biodegradability (also known as biodegradation)--the ability to be disintegrated by biological approaches, including bacteria, fungi, and cells—is another significant property of biomaterials today. Biodegradable materials can minimize the pain of patients from surgeries, especially in tissue engineering, there is no need of surgery in order to remove the scaffold implanted. Wang et al.[81] showed the in vivo degradation of silk via aqueous 3-D scaffolds implanted into Lewis rats. Enzymes are the means used to achieve degradation of silk in vitro. Protease XIV from Streptomyces griseus and α-chymotrypsin from bovine pancreases are the two popular enzymes for silk degradation. In addition, gamma-radiation, as well as cell metabolism, can also regulate the degradation of silk.

Compared with synthetic biomaterials such as polyglycolides and polylactides, silk is obviously advantageous in some aspects in biodegradation. The acidic degraded products of polyglycolides and polylactides will decrease the pH of the ambient environment and thus adversely influence the metabolism of cells, which is not an issue for silk. In addition, silk materials can retain strength over a desired period from weeks to months as needed by mediating the content of beta sheets.

Genetic modification

Genetic modification of domesticated silkworms has been used to alter the composition of the silk.[82] As well as possibly facilitating the production of more useful types of silk, this may allow other industrially or therapeutically useful proteins to be made by silkworms.[83]

Cultivation

Silk moths lay eggs on specially prepared paper. The eggs hatch and the caterpillars (silkworms) are fed fresh mulberry leaves. After about 35 days and 4 moltings, the caterpillars are 10,000 times heavier than when hatched and are ready to begin spinning a cocoon. A straw frame is placed over the tray of caterpillars, and each caterpillar begins spinning a cocoon by moving its head in a pattern. Two glands produce liquid silk and force it through openings in the head called spinnerets. Liquid silk is coated in sericin, a water-soluble protective gum, and solidifies on contact with the air. Within 2–3 days, the caterpillar spins about 1 mile of filament and is completely encased in a cocoon. The silk farmers then heat the cocoons to kill them, leaving some to metamorphose into moths to breed the next generation of caterpillars. Harvested cocoons are then soaked in boiling water to soften the sericin holding the silk fibers together in a cocoon shape. The fibers are then unwound to produce a continuous thread. Since a single thread is too fine and fragile for commercial use, anywhere from three to ten strands are spun together to form a single thread of silk.[84]

Animal rights

As the process of harvesting the silk from the cocoon kills the larvae by boiling them, sericulture has been criticized by animal welfare and rights activists.[85]Mohandas Gandhi was critical of silk production based on the Ahimsa philosophy which led to promotion of cotton and Ahimsa silk, a type of wild silk made from the cocoons of wild and semi-wild silk moths.[86]

^Hill, John E. (2009) Through the Jade Gate to Rome: A Study of the Silk Routes during the Later Han Dynasty, 1st to 2nd Centuries CE. BookSurge, Charleston, South Carolina. ISBN978-1-4392-2134-1. Appendix A: "Introduction of Silk Cultivation to Khotan in the 1st Century CE," pp. 466–467.